1. Field of the Invention
This invention generally relates to electro-optical readers for reading indicia such as bar code symbols and to digital cameras and, more particularly, to imaging arrangements for, and methods of, imaging targets at plural focal planes arranged along an axis away from a two-dimensional, solid-state sensor array.
2. Description of the Related Art
Both charge-coupled device (CCD) and complementary metal-oxide semiconductor (CMOS) image sensors are used in digital cameras for imaging a target. Each such two-dimensional, solid-state sensor has a multitude of cells or pixels for converting incident light into electrons. In a CCD sensor, an accumulated charge of each cell is transported across, and read out of, the array. In a CMOS sensor, several transistors at each pixel amplify and move the charge using traditional wires. CCD sensors have excellent light sensitivity and produce high quality images, whereas CMOS sensors have lower sensitivity, but are much less expensive in manufacture and have an extended battery life. The two-dimensional, solid-state sensor in a digital camera is used with an objective lens assembly, typically focused at infinity in order to capture an image of the target located away from the camera, typically more than two feet away.
Targets, such as optical codes, typically one- or two-dimensional bar code symbols, can also be electro-optically read by readers using two-dimensional, solid-state sensors, preferably in combination with an objective lens assembly which includes one or more focusing lenses and a light collection aperture through which light reflected from the targets is collected. Such readers have a single sharp focus or focal plane located relatively close to the readers (within a few inches of a window on the reader) and a limited depth of field in which a symbol is readable at the focal plane. In order to increase the depth of field of such readers, the aperture is kept relatively small. However, this limits the amount of the light collected, increases the integration time and renders a hand-held reader more susceptible to hand jitter. The versatility and working range of such readers are therefore compromised.